1. Field of the Invention
This invention relates to attrition resistant alumina catalyst supports. More particularly, this invention relates to fluidizable alpha-alumina catalyst supports having improved attrition and high temperature resistance. The alpha-alumina supports of the present invention can be doped or impregnated with appropriate active metal(s) to obtain desired catalytic properties.
2. Background
Those skilled in the art have long recognized the advantages of fluid-bed catalytic processes over fixed-bed catalytic processes. Such advantages include improvement of temperature control and heat transfer, resulting in greater reactor efficiencies. The activity, efficiency, stability, and durability of a catalyst in a fluidized-bed catalytic reaction depend, to a large degree, upon the structural and physical properties of the catalyst support material. A problem, however, with the use of certain support materials is attrition of the support particles by abrasion of the surface of the support particle or fracture of the support particle itself. Excessive particle attrition is caused, for example, by particle to particle contact, abrasion with bed walls and bed internals, as well as distributor jet impingement and abrasion in circulation conduits leading to and from the reactor bed. High particle attrition contributes to product contamination, catalyst loss, plugging of downstream equipment, high filtration costs, and unstable fluidization behavior such as channeling, slugging or increased entrainment of reactants. The deleterious effects of fluidized-bed operations are exacerbated by high temperature conditions.
It has been well-known that aluminum oxide (Al.sub.2 O.sub.3) is an excellent support material for catalysts in a wide range of chemical reactions. Various forms of aluminum oxide (hereinafter referred to as alumina) occur in nature and many have been produced synthetically. Among the conventional catalyst supports for use in catalytic processes, most have been produced from gamma-alumina which is generally characterized by having high surface area, low bulk density, and high mechanical strength. Under high temperature conditions, however, gamma-alumina undergoes changes through various crystalline phases (e.g., delta, eta, theta, kappa, chi, and rho), ultimately transforming into alpha-alumina. Alpha-alumina, the final product of aluminum oxide thermal transformation, is chemically and thermally stable. Because of its thermal and chemical stability, it would be highly desirable to utilize alpha-alumina as a catalyst support in high temperature catalytic processes. However, the aforementioned crystalline transformation is accompanied by a large reduction in surface area with the formation of irregularly shaped, brittle particles that are highly subject to attrition, and an almost complete loss of mechanical strength. Consequently, alpha-alumina has found little use in high temperature catalytic processes, particularly as a carrier or support material for fluid-bed catalysts.
In the past, attempts have been made to produce alpha-alumina with sufficient strength and attrition resistance in order to take advantage of its inherent thermal and chemical stability properties. It is known that the attrition resistance of a support such as a fluid-type catalyst support can be increased by incorporating binders into and onto the catalyst support matrix. The use of binders, however, introduces additional entities into and onto the support that may have their own reactivities, resulting in competing side reactions. A further disadvantage with binders is that they can decrease the surface area, increase the bulk density, and decrease the pore volume of a catalyst support. In addition, most binders will not have sufficient thermal stability to be useful in many high temperature catalytic processes.
The commercial utility of catalyst compositions in reactions which involve conditions of high stress (such as high temperatures and/or pressure, especially under fluidized-bed conditions) require support or carrier materials that are highly resistant to abrasion and attrition. Catalyst researchers continue to look for high efficiency catalysts and supports of increased stability, physical strength, and attrition resistance, and that are useful in reactions involving conditions of high stress. Thus, the ability to employ attrition resistant catalysts supported on alpha-alumina in fluidized-beds, particularly, under high temperature conditions without the use of binders would be highly desirable.